Such an apparatus may be constructed, for example, as a vacuum cleaner, the vacuum chamber being provided with a dust bag. In the case of a vacuum cleaner, the temporary increase of the variable motor power to a power level above the nominal motor power is desirable, for example, for removing a thread from a carpet or for cleaning a hard-to-reach crevice. Switching on the more than nominal power will be hereafter referred to as the "boost" function.
Such an apparatus is known inter alia from patent application WO 87/01921,which corresponds substantially to U.S. Pat. No. 4,920,607, the disclosure of which is hereby incorporated by reference and which describes an apparatus constructed as a vacuum cleaner. In order to increase the variable motor power to a power level above the nominal motor power, the vacuum cleaner described in said Patent has a boost function provided by said means, which boost function is operative for a first fixed time interval (10 seconds ) and a second fixed time interval after its initiation. During the first fixed time interval the boost function causes the variable motor power to be increased, the boost function being attended with a production of heat by the electric motor. Since the electric motor as well as the vacuum cleaner may be damaged as a result of the increased heat production, the first fixed time interval covers a period in which the electric motor as well as the vacuum cleaner cannot be damaged as a consequence of a continuous use of the boost function. During the second fixed time interval (20 seconds ) the boost function prevents the variable motor power from being increased, the boost function being attended with a diminished heat production. The second fixed time interval covers a period after which the electric motor as well as the vacuum cleaner cannot be damaged as a result of a subsequent use of the boost function.
A disadvantage of the apparatus described in said patent application is that the increase of the variable motor power is usually discontinued prematurely as a result of the first fixed interval and is usually inhibited too long as a result of the second fixed interval.